This invention relates to a semiconductor device, more particularly to a semiconductor device comprising memory circuits coated with a protective resinous film for shielding the circuits against radioactive rays and thereby eliminating soft errors caused by the rays.
A semiconductor device, on which memory cells, etc., are fabricated, is usually enclosed within a ceramic or plastic package. It is known that ceramic materials are contaminated with uranium and thorium in an amount equal to several parts per million. Plastic package materials include ceramic fillers which are also contaminated with radioactive materials.
The radioactive materials emit radioactive rays. Such irradiation, particularly an .alpha.-particle, induces pairs of electrons and holes in the substrate. Holes are mobile vacancies in the electronic valence structure of a semiconductor that act like a positive electron charge with a positive mass. The irradiation is apt to change the potential of the storage capacitor and circuit-nodes. Consequently, this potential change causes faulty operation of memory circuits, usually called soft errors, particularly in the case of dynamic random access memories.
In order to eliminate such faulty operation of memory circuits, a protective film made of a radiation shielding resin, such as polyimide and silicone, is generally applied on the surface of a semiconductor chip. For example, when the energy of the emitting .alpha.-particle is assumed to be 5 MeV, the resinous film must have a thickness of from 30 to 50 .mu.m in order to inhibit the penetration of .alpha.-particles. This protective film is thicker than other films applied on a semiconductor device, such as silicon oxide film, phospho-silicate glass and so on.
When the resinous material is cured on the substrate, it undergoes a large strain due to the contraction of its volume. In addition, the difference in thermal expansion between the protective resinous material and the substrate material, e.g., polyimide versus silicon, further affects mechanical strain, especially when the chip is enclosed in a ceramic package at a temperature of about 450.degree. C. Because the coefficients of thermal expansion of polyimide, silicone and silicon are 27.times.10.sup.-6 /.degree.C. 50.times.10.sup.-6 /.degree.C. and 2.3.times.10.sup.-6 /.degree.C., respectively, the expansion of the coating resin is at least ten times larger than that of the substrate. This strain is still further increased by an increase in the thickness and coating area of the protective film.
Japanese Patent Application Laid-open No. 55-128851 of J. Shirasu et al. discloses a semiconductor device prepared from a silicon wafer which has all its regions coated by a protective resinous film except the bonding pads region and the scribing region. However, this patent application does not teach dividing the protective resinous film into several portions so as to decrease the area of a single coating portion and, consequently, reduce the amount of the strain resulting after curing between the protective film and the substrate.